Method for bonding contacts to and forming alloy sites on silicone carbide

ABSTRACT

An alloy of silicon and metal (e.g., gold) formed at the eutectic temperature thereof permits the formation of a bonded contact of the metal (e.g., gold) to silicon carbide. Such a contact is useful for securing a wire to the silicon carbide, for forming alloy sites thereon, etc. A nail head bonder with a threadable heated tip may be utilized in the use of this method.

United States Patent Dunlap [45] Apr. 11, 1972 54 METHOD FOR BONDINGCONTACTS 3,028,663 4/1962 lverson et a1 ..29/472.9 ux TO AND FORMINGALLOY SITES 0N 3,075,282 1/1963 McConville ..29/590 UX 3,091,849 6/1963Cohen ..29/472.9 SILICONE cARBIDE 3,228,104 1/ 1966 Emeis ..29/472.9 X[72] Inventor: Howard L. Dunlap, Granada Hills, Calif. 3,298,093 1/1967Cohen ..29/590 X 3,316,628 5/1967 Lang, Jr ..29/473.1 X [73] "8" AlmaCulver CIY, 3,397,451 8/1968 Avedissian m1... ..29/589 Cahf- 3,341,9389/1967 Hoppe el al ..29/589 x [22] Filed; APR 23 9 3,492,719 2/1970Zeitman et a1 ..29/473.1 X

1 PP N041 819,832 Primary Examiner-John F. Campbell AssistantExaminer-Ronald J. Shore 152 0.8. CL ..29/587,29/473.1, 29/504 Mme-14amHaskell and [51] Int. Cl. ..B0lj 17/00, H0l15/04,H0117/60, A T

H0119/08, H011 11/02, H011 11/04, H01115/08 [57] ABSTR C [58] Field 61Search ..29/472.9, 473.1, 589, 590, An alloy of silicon and metal 8- gformed t e eutec- 29/621 504 537 tic temperature thereof permits theformation of a bonded contact of the metal (e.g., gold) to siliconcarbide. Such a con- R f tact is useful for securing a wire to thesilicon carbide, for [56] e erences cued forming alloy sites thereon,etc. A nail head bonder with 21 UNITED STATES PATENTS threadable heatedtip may be utilized in the use of this th d. 2,918,396 12/1959 Hall..29/472.9 ux me 0 2,937,324 5/1960 Kroko ..29/472.9 UX 18 Claims, 1Drawing Figure g ,I /Z M 0- /3 4:47 Pan/98 .Sadec' METHOD FOR BONDINGCONTACTS TO AND FORMING ALLOY SITES ON SILICONE CARBIDE The inventiondescribed herein was made in the performance of work under a NASAcontract and is subject to the provisions of Section 305 of the NationalAeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 435; 42U.S.C. 2457).

The present invention relates to a method for making bonded electricalcontacts on silicon carbide.

With the increasing need to utilize semiconductor devices in hightemperature environments, materials, which will satisfactorily functionwithout deterioration at high temperatures, have been required. One suchmaterial is silicon carbide, whose melting point is approximately 2,700C. and which is relatively insoluble in most substances. The fabricationof semiconductor devices utilizing silicon carbide, however, haspresented difficulties. One serious problem is the difficulty in formingelectrical contacts to precise locations or sites on a silicon carbidesample with ease and without producing a poor contact. Existingprocesses have not satisfactorily overcome these problems.

In one prior process, a silicon carbide wafer, with a pellet of materialplaced at the desired contact site, is placed in a furnace and heated toapproximately 2,000 C. to form an alloy site and thereafter cooled.After properly made alloy sites are formed, a wire lead is attached tothe wafer at the site. However, the pellet might be improperly orinaccurately placed or accidentally moved and, therefore, the contactsite would not be located where desired. Jigs have been utilized to holdthe pellet in place but are cumbersome and are still liable toaccidental movement or to imprecise placement. Even when proper alloysites are formed, the high temperatures required can adversely affectthe silicon carbide wafer or cause undesired penetration thereof by thematerial of pellet. This penetration damages the wafer and may even makeit useless for its intended purpose.

The present invention overcomes these and other problems by providing amethod which can be carried out under atmospheric conditions in a lowtemperature environment to secure a wire lead to silicon carbide, toform alloy sites, and the like. A pulse of heat is utilizedsimultaneously to form an alloy of silicon and metal from the lead attheir eutectic temperature and to cause adhesion of the alloy and thelead to the silicon carbide sample or specimen.

A nail head bonder having a heated tip is preferably utilized in thepractice of the present invention and the wire to be bonded to thesilicon carbide sample is threaded through the tip. From a prior use ofthe bonder, the wire is equipped with a bead. A small particle ofsilicon, used as a wetting agent, is placed on the substrate at thepoint or site where the contact is to be made. When a pulse of heat isproduced by the bonder tip, an alloy is formed from the bead and thesilicon at the eutectic temperature thereof. For a gold-silicon alloy,the tem perature is approximately 370 C. In addition, the siliconcarbide sample is heated, for example, to approximately 100 C., in orderto prevent heat flow out of the bead-silicon-silicon carbide interfacearea. Once the molten alloy is formed, the surface of the siliconcarbide is wetted and affixation of the wire to the silicon carbideoccurs.

In the use of the present inventive method, when a nail head bonder isutilized, some correlation is required between the wire used, thetemperature of the bonder tip, the wetting agent, and the substratesurface. The tip temperature must not be allowed to approach the meltingpoint of the wire; otherwise, the wire would melt within the tip. Thequantity of silicon and the size of metal bead must be such that theeutectic ratio of silicon to the metal permits formation of the alloyonly at interface and not within the bonder tip. For a gold-siliconalloy, the eutectic ratio is 31 atomic percent of silicon to gold.Regardless of whether a nail head bonder is used, the interfacetemperature at the silicon carbide sample cannot be below the eutectictemperature of the wire and wetting agent. Once the proper parametershave been obtained, the wire may be easily placed at any specificdesired site on the silicon carbide sample with a high degree ofaccuracy.

The silicon used as a wetting agent may be placed on the silicon carbidesample manually or by any other method and, in most cases when greataccuracy is required, the silicon can be applied as a pad by aconventional evaporation process or sputtering process in conjunctionwith photo masking methods.

The use of the present invention does not cause the substrate to enterinto the eutectic condition, thereby avoiding penetration of thesubstrate. After the wire has been fixed in position, it may be used ina variety of ways such as a mechanical hold-down device or as electricalcontacts to silicon carbide of sufficiently low carrier concentration,in the realm of 10 carriers per cubic centimeter, or as alloy sites. Inthe latter use the wire lead is removed and the wafer is heated to asufiiciently high temperature to permit penetration of the wafer by themolten metal. For this use as an alloy site, if the wetting agent hasbeen predoped or if multilayers of materials have been applied, thisalloying operation into the substrate can be efiected without the needfor jigs to locate the defined site. In addition, except for alloysites, once the contact has been formed, it can be removed mechanically,electronically, or chemically and reapplied, if needed or desired,without changing the characteristics of the silicon carbide samplebecause no penetration thereof has occurred. Mechanical removal can beeffected by cutting, electronic by ion or electron beam machining, andchemically by such reagents as aqua regia.

It is, therefore, an object of the present invention to provide a methodfor forming bonded contacts on silicon carbide samples.

Another object is the provision of a simple, yet accurate, method formaking such silicon carbide contacts.

Another object is to provide such a method whereby the silicon carbidesample is not deleteriously affected by the formed contact.

Another object is to provide a method whereby a contact on siliconcarbide, after having been made, may be removed and relocated to anotherposition on the sample.

Other aims and objects as well as a more complete understanding of thepresent invention will appear from the following explanation ofexemplary embodiments and the accompanying drawing thereof, in which:

The FIGURE depicts a preferred apparatus for carrying out the presentinvention.

Accordingly, a nail head bonder 10 is provided with a tip 12 which isadapted to be heated. A feeder hole 14 is provided in tip 12 so that awire 16 may be threaded and fed therethrough from bonder 10. A bead 18terminates wire 16 and is generally formed by a prior use of the bonder.A silicon carbide wafer or sample 20 is shown placed on a work surface22 and a pad or particle of silicon 24 is placed on the sample inreadiness for formation and bonding of the contact. An electrical coil26 is placed adjacent to sample 20 in order to heat the sample; however,other suitable heating means may be utilized.

In operation, when it is desired to join wire 16 to sample 20, bead 18is placed in contact with pad 24 and a pulse of heat, formed by bonderl0 and tip 12, produces an alloy between bead l8 and pad 24 at theireutectic temperature. Upon cooling of the alloy, a bond exists betweenwire 16 and sample 20.

EXAMPLE A silicon carbide specimen of crystal quality, with designatedsites at which contacts were to be made, was placed in readiness forbonding of a gold wire. The wafer may have already undergone priorprocessing, such as by ion implantation or thin layer diffusion. Asilicon chip in the form of a flake was placed on the sample. A goldwire of 1 mil thickness having a bead of approximately 2 mil diameterhad been extended from the bonder tip from a prior use of the bonder.The silicon carbide wafer was heated to approximately C. and the bead ofgold was brought into contact with the silicon chip and pressed thereto.Heat was applied by the tip and, at the eutectic temperature of gold andsilicon, which was estimated at approximately 370 C., an alloy wasformed to bond the gold wire to the silicon carbide wafer.

Testing of the bonded contact thereby made showed that no discemablepenetration of the wafer occurred, that the bond was mechanicallystrong, and that electronic noise was not present. The contact was madeat the precise site desired.

Although the invention has been described with reference to particularembodiments thereof, it should be realized that various changes andmodifications may be made therein without departing from the spirit andscope of the invention.

What is claimed is:

l. A method for forming a bonded electrical contact of a gold wire leadto a silicon carbide crystal without the need of special holdingfixtures comprising the steps of:

placing silicon on the crystal,

placing the wire lead in contact with the silicon, and

heating the wire lead and the silicon in an ambient atmosphericenvironment to a temperature sufficient to form a gold-silicon alloy atthe eutectic temperature to bond the lead to the crystal.

2. A method as in claim 1 further including the step of heating thecrystal.

3. A method as in claim 1 further including the step of utilizing a nailhead bonder having a heating tip for supplying a heat pulse toaccomplish said heating step.

4. A method of forming bonded electrical contacts of wire leads to asilicon carbide specimen without the use of special holding fixturescomprising the steps of:

placing silicon on the silicon carbide specimen at the sites where thecontacts are to be made,

placing the leads in contact with the silicon at the sites, and

forming an alloy of the silicon and the material of the lead on thespecimen at each site to bond the lead to the specimen in an ambientatmosphere.

5. A method of fabricating a bonded electrical contact between a wirelead and a silicon carbide specimen comprising the step of forming analloy of the lead material and silicon on the specimen at the sitethereon where the contact is to be made, said forming step beingperformed under ambient atmospheric conditions at a temperaturesufficient to form the alloy and, consequently, to bond the lead to thespecimen.

6. A method as in claim 5 including the step of utilizing predopedsilicon.

7. A method as in claim 5 including the step of utilizing a photo maskedspecimen.

8. A method as in claim 5 wherein said forming step comprises the stepsof placing silicon on the specimen, locating an end of the lead on thesilicon, and heating the lead and the silicon to the eutectictemperature thereof.

9. A method as in claim 8 wherein said placing step includes the step ofattaching the silicon particle to the specimen.

10. A method as in claim 8 wherein said placing step consists of thesteps selected from the methods of sputtering, evaporation and manuallypositioning the silicon on the specimen.

11. A method as in claim 8 wherein said heating step comprises the stepof utilizing a nail head bonder having a wire lead feed and a heatingtip for supplying a heat pulse to the silicon particle and an end of thewire lead.

12. A method as in claim 5 further comprising the step of heating thespecimen.

13. A method as in claim 5 further comprising the step of pressing thealloy to the specimen.

14. A method as in claim 8 further comprising the step of forming a beadof the wire at the end thereof.

15. A method for bonding gold to a silicon carbide wafer at a specifiedsite thereon including the steps of placing silicon on the wafer at thesite,

placing the gold on the silicon,

heating the wafer to approximately 100 C., and

heating the silicon and gold bead to their eutectic temperature to forma bond between the gold and the wafer under ambient atmosphericconditions. 16. A method as in claim 15 including the step of forming asilicon-gold utectic ratio of 31 atomic percent of silicon.

17. A method as in claim 15 further including the step of removing thebond from the wafer.

18. A method of forming alloy sites on a metal silicon carbide specimencomprising the steps of forming on the specimen at each of the sites analloy of the metal and silicon at the eutectic temperature thereof underambient atmospheric conditions and further heating the specimensufficiently to cause penetration of the wafer by the alloy and to forma bond therebetween.

1. A method for forming a bonded electrical contact of a gold wire leadto a silicon carbide crystal without the need of special holdingfixtures comprising the steps of: placing silicon on the crystal,placing the wire lead in contact with the silicon, and heating the wirelead and the silicon in an ambient atmospheric environment to atemperature sufficient to form a gold-silicon alloy at the eutectictemperature to bond the lead to the crystal.
 2. A method as in claim 1further including the step of heating the crystal.
 3. A method as inclaim 1 further including the step of utilizing a nail head bonderhaving a heating tip for supplying a heat pulse to accomplish saidheating step.
 4. A method of forming bonded electrical contacts of wireleads to a silicon carbide specimen without the use of special holdingfixtures comprising the steps of: placing silicon on the silicon carbidespecimen at the sites where the contacts are to be made, placing theleads in contact with the silicon at the sites, and forming an alloy ofthe silicon and the material of the lead on the specimen at each site tobond the lead to the specimen in an ambient atmosphere.
 5. A method offabricating a bonded electrical contact between a wire lead and asilicon carbide specimen comprising the step of forming an alloy of thelead material and silicon on the specimen at the site thereon where thecontact is to be made, said forming step being performed under ambientatmospheric conditions at a temperaturE sufficient to form the alloyand, consequently, to bond the lead to the specimen.
 6. A method as inclaim 5 including the step of utilizing pre-doped silicon.
 7. A methodas in claim 5 including the step of utilizing a photo masked specimen.8. A method as in claim 5 wherein said forming step comprises the stepsof placing silicon on the specimen, locating an end of the lead on thesilicon, and heating the lead and the silicon to the eutectictemperature thereof.
 9. A method as in claim 8 wherein said placing stepincludes the step of attaching the silicon particle to the specimen. 10.A method as in claim 8 wherein said placing step consists of the stepsselected from the methods of sputtering, evaporation and manuallypositioning the silicon on the specimen.
 11. A method as in claim 8wherein said heating step comprises the step of utilizing a nail headbonder having a wire lead feed and a heating tip for supplying a heatpulse to the silicon particle and an end of the wire lead.
 12. A methodas in claim 5 further comprising the step of heating the specimen.
 13. Amethod as in claim 5 further comprising the step of pressing the alloyto the specimen.
 14. A method as in claim 8 further comprising the stepof forming a bead of the wire at the end thereof.
 15. A method forbonding gold to a silicon carbide wafer at a specified site thereonincluding the steps of placing silicon on the wafer at the site, placingthe gold on the silicon, heating the wafer to approximately 100* C., andheating the silicon and gold bead to their eutectic temperature to forma bond between the gold and the wafer under ambient atmosphericconditions.
 16. A method as in claim 15 including the step of forming asilicon-gold utectic ratio of 31 atomic percent of silicon.
 17. A methodas in claim 15 further including the step of removing the bond from thewafer.
 18. A method of forming alloy sites on a metal silicon carbidespecimen comprising the steps of forming on the specimen at each of thesites an alloy of the metal and silicon at the eutectic temperaturethereof under ambient atmospheric conditions and further heating thespecimen sufficiently to cause penetration of the wafer by the alloy andto form a bond therebetween.